The invention relates generally to electromagnetic railgun accelerators and more particularly to the formation of an electrically conducting plasma arc which can be used as an armature to accelerate a projectile in a railgun.
Railgun accelerators have potential to accelerate projectiles to very high velocities in very short distances. Recent research has demonstrated the usefulness of railguns for research and industrial applications. The railgun accelerator is essentially a DC motor consisting of a pair of rigid parallel conductors that carry current to and from an interconnecting movable conductor. The connecting link functions as an armature and the parallel rails serve as a single turn field winding. The resulting Lorentz force on the armature is proportional to the square of the current. Railguns will enable research and applications at previously unattainable velocities, in excess of 10 km/s. Potential applications include (1) equation of state research of matter at high pressure and high energy density, (2) orbital launching from the earth's, moon's and other planetary surfaces, and (3) fusion energy production, either to inject fuel pellets into a magnetic fusion reactor or to ignite fuel pellets with the impact of small projectiles.
It is not necessary that the armature be a soid metal conductor. An arc discharge initiated across the base of a dielectric projectile can also act as an armature if it is confined behind the projectile. The confinement can be provided by the conducting rails on two sides and dielectric rail spacers on the other two sides. The plasma arc armature is in fact preferable to a solid metallic conductor since it removes the limitations resulting from sliding contacts and permits higher velocities. The plasma easily maintains contact with the rails. A variety of power sources have been utilized to provide the high current and long pulse durations needed to accelerate projectiles to hypervelocities including batteries, capacitor banks, homopolar generators and magnetic flux compression generators.
In a typical prior art railgun system, as shown in FIG. 1, a projectile is injected into the railgun by a gas gun and a capacitor bank C.sub.o generates a current through the railgun circuit via a fuse mounted on the back of the projectile. The overloaded fuse rapidly vaporizes, however, and establishes the initial plasma arc. The arc and the projectile accelerate along the rails of the railgun. To avoid excessive launcher damage the projectile is sometimes injected into the breech of the railgun with a helium gas gun. By pre-accelerating the projectile to about 1 km/s with a gas injector the dwell time of the arc on the rails and dielectric is reduced. By reducing the dwell time of the arc at the projectile starting position, rail and dielectric erosion are greatly reduced.
Railgun operation for equation of state, space and other applications requires evacuation of the launcher so that the projectile is the first significant pressure generator at the target. An initial launcher pressure of about 0.001 atmosphere (0.76 torr) or less is often needed. The utilization of an evacuated launcher can result in formation of a spurious arc in the region in front of the projectile. The use of a gas gun injector can further aggravate this problem. If the full accelerating high voltage is applied to the rails to produce the plasma arc, residual vapor pressure in the railgun and gas leakage past the projectile can produce a gas pressure reducing the breakdown voltage in front of the projectile. Arc breakdown in front of the projectile results in loss of performance, interference with the experiment and produces more severe erosion of the launcher. It is difficult, but possible, to prevent gas leakage past the projectile until a low voltage plasma arc is fully formed behind the projectile. It is often not possible to eliminate all residual gas from the barrel bore, hence the chances of having a zone of gas preceed the projectile is high. In a launcher initially evacuated to the low pressure side of Paschen's breakdown relation, the preceeding gas will tend to cause breakdown at the minimum voltage, typically lower than the power supply voltage.
Accordingly, it is an object of the invention to provide a method of railgun arc formation without high rail voltage.
It is also an object of the invention to provide railgun arc formation in a high vacuum launcher in which the projectile is injected by means of a gas gun.
It is a further object of the invention to provide a method for railgun arc formation which greatly reduces rail and dielectric erosion.